Corrosion is an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When the electrons flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust.
A cathodic protection system is implemented in water heaters to prevent corrosion of the water heater tank. The cathodic protection system includes an anode rod, which is electrically connected to the metal water heater tank. The anode rod is comprised of a metal, such as aluminum, magnesium, zinc, or alloys, that is more active than the metal tank of the water heater. The water heater tank is generally comprised of glass coated steel.
When water is introduced into the water heater tank, a galvanic circuit is created between the metal tank (and/or connectors) and the anode rod. As a result, electrical current flows from the anode, through the water, to the cathode, thus, the anode rod begins to corrode. If the water supply has a high mineral content, the current flow will increase, resulting in a corresponding increase in the consumption of the anode rod.
The addition of a resistor in the galvanic circuit can reduce the consumption time of the anode. Current resistored anodes have utilized electronic component type resistors that are costly, relatively difficult to assemble, and the assembly is fragile. A fragile resistor connection can result in a loss of ground connection that disables the anode and results in accelerated corrosion failure of the water heater tank.
Occasionally, certain natural waters that are heated in the presence of a magnesium or aluminum anode can generate smelly water with a sulphur aroma that is undesirable. The addition of a resistor in the galvanic circuit can reduce the anodic current to address the smelly water issue while not detrimentally impeding the necessary current flow that is important for proper water heater tank passivation protection.